In the related art, it is not considered that an antenna for a broad bandwidth is used in a situation in which directivity is required. However, recently, the situation in which a broadband antenna with directivity is required has increased. In order for the broadband antenna to have an appropriate directivity, a reflection plate which reflects an electromagnetic wave is generally used. The reflection plate is provided in a location which is generally separated from the antenna by λ/4 (λ is a wavelength of an electromagnetic wave which is used) (refer to, for example, Patent Literature 1). That is, when an antenna element and a ground element (ground plate) are combined together to operate, for example, in a case where antenna characteristics such as emission efficiency or gain increase, setting of a gap between the antenna element and the ground plate is very important.
Specifically, if a material of the ground element is assumed to be a complete electric conductor, a condition for obtaining the best antenna characteristics is that a gap between the antenna element and the ground element has a length of a quarter of a wavelength of a wave which is used. In order to satisfy the condition, the antenna has a limitation of minimizing a size thereof.
Accordingly, a low profile antenna which employs a structure of an artificial magnet conductor that is called an electromagnetic band gap (EBG) structure is proposed. That is, the EBG structure is a structure in which unit cell patterns of a square shorter than an emission wavelength of an antenna are arranged in a matrix. The unit cell patterns formed of a metal are formed on a surface of a dielectric substrate which configures the artificial magnet conductor, a ground metal plate is formed on a rear surface of the dielectric substrate, and an artificial magnet conductor which is close to a complete magnetic body and has high surface impedance is formed (refer to, for example, Patent Literature 2).
As described above, a method for designing an artificial magnet conductor which reflects a predetermined frequency by mainly using the artificial magnet conductor for the reflection plate is disclosed (refer to, for example, Non Patent Literature 1 and Non Patent Literature 2).
Non Patent Literature 1 discloses a method for appropriately designing a distance between a frequency selective surface (FSS) and a ground plate, in an artificial magnet conductor in which there is air (εr=1) between the FSS and the ground plate.
Non Patent Literature 2 describes design of an artificial magnet conductor according to an FSS which uses a dielectric layer.